There are many monitoring systems where an identity label is attached to an object, and where the position of the object is determined by means of wireless communication with the identity label The motion of the object may he estimated by detecting the position of the object at different points of time.
Active and passive RFID are examples of a technique which is used with identity labels.
Passive RFID means, that an RFID-chip receives a radio-frequency (RF) reading signal and uses the energy in the reading signal to return its ID and possibly other data. This technique may for example be used for tracing of luggage on an airport by integrating an RFID-chip into a tag which is attached to luggage. Readers positioned along a conveyor belt can read the RFID-chips, and use the information to route the luggage to the right aircraft. A type commercially available passive RFID-units has about 0.5 mm diameter and has antennas of a few cm. Both chip and antennas may have a thickness of a few tenths of a mm. Such RFID units can be delivered on film which can be stuck on a surface. Passive RFID are also used among others to identity labelling of animals by implanting a chip the size of a rice grain below the skin of the animal. The unique identity of the RFID chip may later be read out, for instance at a veterinarian, and being associated with the owner and other data in a database. Passive RFID-chips may thus be relatively small sized, and therefore are relatively easy to hide in or on an object. However, the range is a substantial constraint for use of passive RFID. The range for passive RFID depends on the power of the reading signal, and is usually in the magnitude of a few meters or less.
With active RFID, the RFID-chip has a power supply of its own. The range is thus not limited by the power of an incoming signal. Instead active RFID may need more energy than passive RFID, which means larger and heavier batteries or power supply via a cable.
GPRS (General Packet Radio System) and GPS (Global Positioning System) are examples of systems that can be used to determine the position of an object.
GPRS hearing is a service which is offered by major mobile network owners, and is performed in that a mobile unit is connected to up to three base stations in the mobile network. The base stations determine the signal strength of the mobile unit As the network owner knows the exact position of the base stations, it is possible to determine the position of the mobile unit by triangulation. GPRS-positioning assumes that the mobile unit is switched on and connected to at least one base station. The positioning precision depends on the spacing between the base stations, and varies in today's GSM-network in Norway from about hundred meters in densely populated areas to a few kilometers in rural areas. Corresponding systems based on triangulation of a mobile unit may be implemented in different mobile networks, and it is expected that other systems will be preferred if, when or where the density of base stations is greater than in today's GSM-network in Norway, It should also be appreciated that “triangulation” hereinafter is not limited to GPRS, but also comprises well-known bearing methods with hand-hold directive antennas which are independent of mobile networks.
GPS is a positioning system in which satellites with known orbital tracks broadcast radio signals with information about their track and precise information about when the signal was transmitted. A GPS receiver on the earth surface which receives signals from multiple satellites can use this information to determine its own position. The GPS receiver can calculate its position with precision of about 7 m without corrective signals. The precision may be improved to about 2 m with corrective signals, and to some cm with special equipment. Iridium is a second satellite based positioning system with similar properties. Hereinafter “satellite based positioning system” is used for GPS, Iridium and other satellite based positioning systems. Common for systems is that they have a precision of about 10 m or less, and that the broadcasted radio signals of the satellites must be able to reach a receiver.
Battery-powered tracing units which are hidden on or in an asset may for instance be used to locate an automobile, a boat, luggage or other asset after theft. Battery-powered tracing units may also be used to find a pet that has escaped or in a “fetter” with a tracing unit being used to check if a convicted person is located within a defined area.
Chipset and modules for the techniques described so far are commercially available, and prices and dimensions are constantly reduced.
A general problem with battery-powered units is as mentioned that the amount of energy which can be stored in a battery is limited. It is therefore common to let the electronics go into hibernation when it is not in active use. US 2004/034470 A1 describes a tracing unit that has such a hibernation state in order to save energy, and which is activated by motion. Motion activation may be suitable for then alarm, but does not fit to trigger wake-up of a tracing unit on an animal which constantly is in motion. Motion activation of a theft protection on, for instance, an automobile, bicycle or boat requires in addition that the user must remember switching off the theft protection prior to he/she moves the automobile, the bicycle or the boat. There is thus a need for a unit that is not activated by motion.
Norwegian patent NO 326999 B1 describes a system for monitoring animals where a collar is laid around the neck of an animal. The collar comprises a GPS unit for position finding, and means for two-way communication with a peripheral communication unit. The system further comprises a communication unit for activation of and reception of signals from a passive probe implanted into the body of the animal. The probe detects one or more physiological parameters in the animal and communicates these parameters to the communication unit. Alarm is given if a physiological parameter differs from predetermined values. The alarm may be an acoustic signal which is given by means of a speaker integrated into the collar. This signal is intended to temporarily scare predators, and it is disclosed to be essential that a human arrives at the location shortly after the alarm has occurred. For this objective the alarm can also being sent as a radio signal to the peripheral communication unit.